This invention relates to a torque split control apparatus for use with a 4WD automotive vehicle supported on a pair of primary drive wheels and a pair of secondary drive wheels.
For example, U.S. Pat. No. 5,132,908 discloses a torque split control apparatus for use with an automotive vehicle. The conventional torque split control apparatus is arranged to transmit the drive from the engine to the primary drive wheels (rear road wheels) of the vehicle and to the secondary drive wheels (front road wheels) of the vehicle through a torque distributing clutch capable of varying a torque transmitted to the secondary drive wheels. It is the current practice to control the torque distributing clutch to adjust the transmitted torque in proportion to the degree to which the rear road wheels spin by increasing the transmitted torque as the front and rear road wheel rotational speed difference increases and decreasing the transmitted torque control gain, which is proportional to the front and rear road wheel rotational speed difference, as the vehicle lateral acceleration increases. The control gain is determined in inversely proportion to the vehicle lateral acceleration. This is effective to provide a greater control gain for a road surface having a low friction factor and a small control gain for a road surface having a high friction factor so as to provide a neutral steer characteristic regardless of the road surface friction factor.
With the conventional torque split control apparatus, however, the control gain is determined only by the vehicle lateral acceleration regardless of whether the vehicle is starting or the vehicle is moving in a straight or curved path. If the control gain is set at a greater value for the vehicle lateral acceleration to provide a sufficient traction performance when the vehicle is starting, a great torque WILL be transmitted to the front road wheel so as to suppress the rear wheel spin even with a great degree of depression of the accelerator pedal when the vehicle is moving on a road surface having a low friction factor. As a result, the driver will continue to operate the vehicle without any indication that the road surface has a low friction factor. If the control gain is set at a smaller value for the vehicle lateral acceleration to provide a feel of rear road wheel spin in response to an accelerator pedal operation so as to provide an indication to the driver when the vehicle is moving on a road surface having a low friction factor, however, the rear road wheels will spin so that a sufficient traction performance cannot be obtained when the vehicle is starting on a road surface having a low friction factor.